This invention relates to a solenoid switch for a coaxial engine starter and more particularly to a coil bobbin structure of a solenoid coil for actuating switch contacts of a solenoid switch.
FIGS. 1 and 2 illustrate one example of a conventional coaxial engine starter. The coaxial engine starter typically comprises a d.c. electric motor 1 having a hollow armature rotary shaft 2, a solenoid switch 3 disposed at the rear end of the d.c. motor 1 and having a plunger 27 with a plunger rod 3a on which a movable contact 28 is mounted, and a push-rod 4 supported from the plunger rod 3a of the solenoid switch 3 and inserted into the armature rotary shaft 2. The armature rotary shaft 2 has an output shaft 5 coaxially inserted therein from its front (right-hand end as viewed in FIG. 1) end so that the rear end of the output shaft 5 is in engagement with the front end of the push-rod 4. A pinion 21 is disposed at the front end of the output shaft 5. When solenoid switch 3 is energized, the plunger 27 is magnetically driven forward (toward the right as viewed in FIG. 1) and the pinion 21 on the output shaft 5 is pushed forward into engagement with an engine ring gear (not shown) through the plunger rod 3a and the push rod 4.
At the rear end of the armature rotary shaft 2a, there is mounted a commutator 6 around which electrical brushes 9 held by holders 8 secured to a mounting plate 7 are brought into slidable contact. Behind the commutator unit including the commutator 6, the brushes 9 and the holders 8, and at a front end of the solenoid switch 3, a substantially disc-shaped core plate 10 is disposed. The core plate 10 has formed therein a central bore 11 (FIG. 2) and a magnetic attraction, force generating portion 12 axially and circumferentially extending on one side of the plate 10 along the edge of the central bore 11. The core plate 10 also has integrally formed therein, on the side opposite to the attraction force generating portion 12, a cylindrical portion 13 axially extending from the edge of the central bore 11. The cylindrical portion 13 rotatably supports the rear end of the armature rotary shaft 2 through a sleeve bearing.
On the rear surface of the core plate 10 on which the attraction force generating portion 12 is provided, a pair of stationary contacts 14 and 15 are disposed so that they are brought into electrical engagement with the movable contact 28 when the plunger 27 is actuated. The stationary contact 14 has an integral connection portion 16 extending through a through hole 17 and connected to a connection plate 19 connected to a terminal bolt 18 on the front side of the core plate 10. The stationary contact 15 has an integral conductor portion (not shown) which is connected to the positive-side brush 9 as well as to the lead-in conductor of the excitation coil 20 of the solenoid switch 3. The stationary contacts 14 and 15, the conductor portions and the connection plate 19 are molded with a resin within the through hole 17 of the core plate 10.
The coaxial engine starter also comprises a planetary speed reduction gear 22 mounted at the front end of the armature rotary shaft 2 of the d.c. motor 1 for reducing the speed of the armature rotary shaft 2 and an over-running clutch assembly 23 connected between the planetary speed reduction gear 22 and the front end of the output rotary shaft 5 for transmitting the speed-reduced output of the planetary speed reduction gear 22 and for preventing the d.c. motor 1 from being driven by the engine when the output rotary shaft 5 is rotated at a high speed by the engine.
In the conventional coaxial engine starter, the solenoid switch 3 is connected to the rear end of the d.c. motor 1 through a plurality of bolts 24, which extend through an annular space 3c defined between the tubular case 3b and the outer surface of the excitation coil 20 as best seen from FIG. 2, so that the outer diameter of the excitation coil 20 cannot be made larger than a certain limit. Therefore, in order to provide a necessary space for accommodating the winding of the excitation coil 20, the thickness of the cylindrical portion 25a of the coil bobbin 25 on which the excitation coil 20 is wound must be made thin. Also, the resinous holder member 26 molded around the stationary contacts 14 and 15 which are positioned at the end portion of the coil bobbin 25 has a very thin wall 26a between the coil bobbin cylinder 25a and the stationary contacts 14 and 15. This arrangement is disadvantageous in that the electrical insulation at the thin wall 26a is insufficient for a relatively large current flowing through the stationary contacts 14 and 15. Also, the formation of the thin wall 26a of the resinous holder member 26 is difficult.